The rehabilitation protocols in knee osteoarthritis are often difficult to be applied in elderly patients because they are long lasting and need the constant participation of patients. To rapidly improve the motor performances of patients it is possible to use the mechanical vibration applied to individual muscles using a protocol called "repeated muscle vibration" (rMV). The purpose of this single-blind randomized placebo-controlled study is to evaluate the effectiveness of the repeated muscle vibration (rMV) in terms of increasing the ability of the patients affected by knee osteoarthritis, compared to an ineffective treatment, considered as a placebo. Each subject, randomly assigned to either the group 1 (study group) or placebo-treatment (group 2)is administered 3 daily applications of rMV of 10 minutes each, for 3 consecutive days. Between two successive applications it's observed a break of at least 15 seconds. The probe of the specific instrument (Cro ® System) is placed near the supero-medial margin of the patella, on both quadriceps. Patients in group 2 (control group) are subjected to a treatment with muscle released in which the probe of the same instrument is approached to the quadriceps, without making contact. The instrument in these conditions emits a buzz but not provokes muscle vibration. The primary outcome of the study is to determine the changing in patients' ability (as measured by WOMAC scale = Western Ontario and McMaster Universities Osteoarthritis Index) . The secondary outcomes of the study are the assessment of changing of patients' balance and risk of falling (as measured by the Tinetti scale) and assessment of changing of patients' quality of life (as measured by EQ-VAS= EuroQuality of Life-VAS scale).Outcome measures are administered at baseline (T0), at the end of the treatment protocol (T1), at 1 month (T2), 3 months (T3) after the end of treatment and 6 months (T4) after the end of the treatment.